Known methods for suppressing chattering vibrations during thread cutting have been disclosed in JP 2004-209558A and JP 2012-56066A. These methods suppress chattering vibrations by changing rotation speed of a main spindle during thread cutting.
In the technique disclosed in JP 2004-209558A, a relative phase error between a main spindle position and a feed axis position during thread cutting is computed at the main spindle position, and the relative phase error is compensated relative to the main spindle position to provide a pseudo main spindle position based on which a movement of the feed axis is computed.
In the technique disclosed in JP 2012-56066A, a time constant, an acceleration/deceleration type, and a positional loop gain are made the same for a main spindle and a feed axis to eliminate a phase error between the main spindle and the feed axis relative to a change of rotation speed of the main spindle.
Both methods disclosed in JP 2004-209558A and JP 2012-56066A are techniques for changing the rotation speed of the main spindle. In this case, it is also necessary to change the speed of the feed axis to provide a constant thread pitch.
In the method disclosed in JP 2004-209558A, however, the acceleration/deceleration time of the feed axis is not changed, so that an amount of acceleration/deceleration delay of the feed axis is changed depending on the speed of the feed axis.
In the method disclosed in JP 2012-56066A, multi-stage acceleration/deceleration can be applied depending on the rotation speed of the main spindle, with the acceleration/deceleration time being longer in a higher speed range. Therefore, as in the method disclosed in JP 2004-209558A, the amount of acceleration/deceleration delay of the feed axis is also changed depending on the speed of the feed axis, with variation of the delay amount being larger than that in JP 2004-209558A.
If the amount of acceleration/deceleration delay of the feed axis is changed according to the speed of the feed axis as mentioned above, a cutting amount may be increased at a thread-cutting-direction changing part, such as a connecting part of thread cutting. Accordingly, there may be a risk of damaging tools due to unexpectedly large cutting load.